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Modeling of the Current Distribution in Aluminum Anodization Rohan Akolkar and Uziel Landau Department of Chemical Engineering, CWRU, Cleveland OH th Meeting of The Electrochemical Society, San Antonio, TX. Yar-Ming Wang and Hong-Hsiang (Harry) Kuo General Motors R&D, Warren MI

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Anodic Oxide Films on Aluminum Current distribution – Significance Kinetics of oxide growth Modeling of Current and Potential Distribution Comparison with experiments Effect of operating conditions (t, V, T) Conclusions Outline

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Aluminum Anodization dc voltage = V Alloy wt. % H 2 SO 4 time = min oxide films ~ 5-25 μm Introduction Al metal Al 2 O 3 barrier Oxide pores 5-25 μm ~30 nm

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Important Issues in Al Anodization Analyze and model the current distribution in anodizing systems, and compare with experimental measurements. Objective Anodized parts with complex, non-accessible features experience large oxide thickness variations. What are the current distribution characteristics inside non-accessible cavities ? How are they affected by the operating conditions ?

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Governing Equations Net Flux = Diffusion + Migration + Convection Boundary Conditions Insulator (zero current) : Electrode (Resistive Oxide) : + _ H+H+ zjzj v Assume : No concentration gradients Steady state Potential Distribution Mott Cabrera Kinetics

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Mott Cabrera Kinetics : i = A exp (B V) A, B: ionic transport parameters within the oxide film Anodization kinetics VERY HIGH SURFACE RESISTANCE leads to VERY HIGH SURFACE OVER- POTENTIALS Increasing temperature

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Oxide Thickness Distribution Current Density : Faraday ’ s law : + _ current efficiency oxide porosity

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Analytical Modeling e.g. analytical solution of current balance equations Numerical Modeling e.g. CELL DESIGN*, FEM, FDM to solve Laplace equation Scaling Analysis e.g. Wagner number : Current and Potential Distribution Methods to compute current distribution * CELL DESIGN, L-Chem Inc., Shaker Heights, Ohio

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_ _ + Parallel plate anode assembly Cathode 30 Cathode Anodes Experimental setup side shields z y x z x z y

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Numerical Modeling Geometry Electrode Properties e.g. kinetics Electrolyte Properties e.g. conductivity Cell Design ’ s BEM* Solver Potential Map Current Distribution Deposit Profile * Boundary Element Method Oxide Properties e.g. porosity

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Simulation Results Potential Distribution Current Distribution Significant potential drop ONLY in the interior of the parallel plates NON- UNIFORM oxide in the interior

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Anode Cathode Measurement of Oxide Distribution Uniform Oxide Non-Uniform Oxide Oxide thickness measured along the anode at ~5 cm intervals for comparison with modeling results

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Experimental vs. Modeling Anodic Oxide Thickness (microns)Distance Along the Electrode (cm) Uniform oxide thickness on the exterior Non-uniform distribution in the interior

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Anodic Oxide Thickness (microns) Distance Along the Electrode (cm) Effect of Anodization Time Constant oxide resistance 15 min 35 min

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Anodic Oxide Thickness (microns) Distance Along the Electrode (cm) Effect of Anodization Time – Distributed resistance Constant oxide resistance Low growth rates for distributed resistance within entire oxide 15 min 35 min

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Effect of Anodization Voltage Anodic Oxide Thickness (microns) Distance Along the Electrode (cm) 14 V 18 V Low oxide thickness inside the interior Uniform oxide

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Effect of Anodization Temperature Anodic Oxide Thickness (microns) Distance Along the Electrode (cm) 15 o C 25 o C Low oxide thickness inside the interior Uniform oxide

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An electrochemical CAD software used to model the current distribution in anodizing. Excellent agreement between modeling and experiments. The oxide growth rates are independent of time indicating a porous oxide growth – the oxide resistance resides in a compact barrier film at its base. Current distribution was highly non-uniform in high aspect ratio cavities due to dominance of ohmic limitations over surface resistance. Main Conclusions

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